1. Field of the Invention
The present invention relates to a diagnostic X-ray apparatus wherein an arm member for supporting an X-ray tube and an image intensifier can be rotated around a vertical axis.
2. Description of the Related Art
A conventional diagnostic X-ray apparatus is shown in FIG. 1. The diagnostic subject of this apparatus is circulatory organs. A patient 14 is laid on a top plate 12 of a bed 10 which is placed on a floor of an examining room or a diagnosis room. An X-ray tube 16 and a detector (including an image intensifier, TV camera, or X-ray film if necessary) 18 are mounted on both ends of a C-shaped arm 20 which is shaped like a semicircular arc so that they are faced to each other with the patient 14 therebetween. An X-ray emitted from the X-ray tube 16 is incident on the detector 18 through the patient 14 and thus an X-ray fluoroscopic image is obtained.
The C-shaped arm 20 is supported by an arm holder 22 which includes an arc portion. The arm holder 22 rotatably slides the C-shaped arm 20 along the arc portion as shown by an arrow A so as to view the patient 14 in an oblique direction other than the front direction, as shown in FIG. 2. The driving source of this slide movement (motor or the like) is provided in the arm holder 22. The axis of this rotational slide is an axis passing through the center of a circle including the arc portion of the arm holder 22.
The arm holder 22 is formed of the arc portion and a horizontal portion which are integrally formed. The horizontal portion supporting the arc portion is horizontally supported by a supporting member 24. The supporting member 24 can rotate the horizontal portion of the arm holder 22 as shown in an arrow B of FIG. 1 and vertically shift the arm holder 22 in the Z direction to adjust the height of the arm holder 22. The driving source of this rotation and movement (motor or the like) is included in the supporting member 24. The axis of the rotation in the direction B, i.e., the horizontal portion of the arm holder 22 crosses the axis of the rotation in the direction A. The crossing point IC is called as an isocenter. If the C-shaped arm 20 is a semicircle arc, the isocenter IC is positioned on the connecting line between the X-ray tube 16 and the detector 18.
The supporting member 24 is suspended from the ceiling of the room via a supporting pole 26. Since the C-shaped arm 20 for supporting the X-ray tube 16 and the detector 18 is considerably heavy, the supporting pole 26 is located on a point which can easily balance the weight of the C-shaped arm 20 and the supporting member 24.
A supporting member 28 for suspending the upper end of the pole 26 is provided in the ceiling. The supporting member 28 can drive or rotate the pole 26 and the C-shaped arm 20 in a .theta. direction as shown in FIG. 3 and move the pole 26 and the C-shaped arm 20 in a lateral direction X (perpendicular to the sheet plane of FIG. 1) as shown in FIG. 4. The driving source of this rotation and movement (motor or the like) is included in the supporting member 28.
The supporting member 28 is further suspended by a supporting member 30 which is provided in the ceiling. The supporting member 30 can move the C-shaped arm 20 in a longitudinal direction Y (lateral direction in the sheet plane of FIG. 1) as shown in FIG. 5. The driving source of this movement (motor or the like) is included in the supporting member 30.
Though not shown, a Y-guide rail is fixed on the ceiling for guiding the movement of the supporting member 30 in the Y direction, and an X guide rail is fixed on the supporting member 30 for guiding the supporting member 28 in the X direction. As a result, the C-shaped arm 22 can be rotated in the .theta. direction and moved in the X and Y directions.
If the supporting pole 26 is rotated in the .theta. direction while the position of the pole 26 is fixed as shown in FIG. 3, the X-ray tube 16 is displaced from the patient 14 and thus it becomes impossible to obtain the fluoroscopic image of the patient 14. Generally, this .theta. rotation is performed while the arm 20 is rotatably slid in the direction A to obtain the fluoroscopic image in an oblique direction. Alternately, the .theta. rotation is performed in order to make room for the operator since the C-shaped arm 20 and the supporting member 24 are considerably large. In any case, it is not preferable to displace the X-ray tube 16 from the patient 14.
Therefore, it is proposed that the pole 26 is moved in the X and Y directions in the horizontal plane if the pole 26 is rotated. According to this proposal, the arm 20 is rotated around a vertical axis including the isocenter IC by driving the pole 26 in the .theta. direction and shifting the pole 26 to a position 26' which is realized by a combination of the X and Y movements, as shown in a broken line of FIG. 6. This type of rotation is called isocentric rotation since the rotation axis passes through the isocenter IC. During the isocentric rotation, the ROI (Region Of Interest) of the patient cannot be outside the field of view of the X-ray tube 16 if the ROI is initially positioned in the field of view.
In the conventional apparatus, the movement of the pole 26 in the X and Y directions in order to perform the isocentric rotation is controlled by an open loop control circuit based on velocity data which is previously calculated in accordance with the angle .theta. of rotation. That is, the velocity of the motors for moving the supporting members 30 and 28 in the Y and X directions is controlled based on the angle .theta. of rotation of the pole 26.
However, the conventional open loop control circuit stops the movement of the pole 26 by merely setting the velocity to 0. Therefore, the isocenter may be displaced when the movement of the pole 26 is stopped. If the pole 26 is rotated/stopped many times, the displacement is accumulated which results in a large deviation of the isocenter. In order to prevent this deviation due to the intermittent rotation, it is necessary to confirm the position of the isocenter before the start of rotation and correct the position of the arm if the isocenter is displaced. This is a troublesome work for the operator.
Further, in the conventional open loop control circuit it is assumed that the moving velocity of the pole is a constant velocity. Therefore, it is not possible to perform the isocentric rotation when the initial angle .theta. of rotation is not 0. If the moving velocity varies due to the change in a load or the amount of movement of the pole 26 exceeds a predetermined level, the isocentric rotation cannot be performed any more.
If the isocentric rotation cannot be performed, the C-shaped arm 20 may be moved unexpectedly. In this case, the operator may be exposed to danger by the supporting member 24 and the arm holder 22.
As described above, the conventional apparatus cannot keep the isocenter at a desired position and accurately perform the isocentric rotation.